The present invention relates to battery monitoring apparatus and more particularly to a battery state-of-charge indicator.
Storage batteries are used in numerous applications where it is important to know the amount of available energy remaining in the battery. For example, a battery powered electric vehicle must rely upon the energy stored in the on-board batteries for propulsion and the replenishing of stored energy requires special equipment which is only available at a charging station. Thus, a means for indicating the energy state of the remaining battery charge can be advantageously used by the vehicle operator to ensure that the vehicle is returned to a charging station before the battery has been completely discharged. The vehicle batteries represent a substantial investment and the amortization of battery costs depends upon the available number of charge/discharge cycles and upon the average depth of discharge of a lead-acid battery. It is well known that the life of a battery is reduced significantly when it is repeatedly discharged such that the specific gravity of the electrolyte falls below 1180. Hence, it is desirable to provide some means for recognizing that the battery state-of-charge is approaching this level.
Prior art systems devised for indicating the energy remaining in a battery and/or detecting a low battery condition have fallen into three broad categories: damped voltmeters, battery condition monitors, and ampere-hour meters. None of these systems have proven entirely reliable or successful.
The use of a heavily damped voltmeter connected directly across the battery is the oldest and most commonly used indicator of the battery state-of-charge. The vehicle operator can often judge the battery charge condition by the magnitude of voltage drop during a specific manuever such as acceleration, but an accurate determination requires a high level of skill and close observation by an operator who is likely preoccupied with vehicular operation. Thus, a voltmeter arrangement is unsatisfactory because it requires a skilled operator for interpretation and in addition requires that a specific load be placed on the voltmeter at the time a reading is taken.
Battery condition monitors have been employed in some battery powered industrial trucks. These devices have a voltage level switch which is activated when the battery terminal voltage drops below a preset level (usually 80-85% of nominal voltage). Should the voltage remain below this level for a preset time interval, typically 15-30 seconds, an indicating lamp is energized and a second interval timer is started. After a second predetermined time interval, a specific work function is disabled so that no additional work can be accomplished, thereby forcing the operator to return to the charging station. A sensing and detection means of this type is inexact and dependent upon many variables, and, as a result, setting of the voltage trip-point and the two time delay intervals must be adjusted by trial and error. In addition, no continuous indicating means is available to advise the operator of the present state-of-charge and the low charge indication will often catch the operator by surprise.
A common type of instrument for continuously displaying the state-of-charge for battery powered vehicles is the ampere-hour meter. Battery-electric automobiles which were popular in the early part of this century included an ampere-hour meter which was reset by the ampere-hours of recharge. This early meter was an electromechanical unit, but more recent designs employ a solid-state version of the ampere-hour meter using a reversible electrochemical plating cell. In the electrochemical cell, charging of the battery plates a material at a rate corresponding to the magnitude of the discharge current so that the total plating is the product of current and time. During recharge, the plating is reversed in a similar fashion. For example, the ampere-hours (AH) available from a lead-acid battery depend upon the rate of discharge. Thus, a battery rated at 300 AH at a current drain which would deplete the charge in 6 hours may only provide 220 AH at a current corresponding to a 1-hour discharge rate so that the ampere-hour meter may indicate that one-third of the energy is available when in fact the battery is completely discharged. In addition, discharged batteries are often exchanged at the charging station for a fully charged set and since the charging history of the new set is unknown, the meter cannot be properly reset. Furthermore, the ampere-hours recoverable from a fully-charged lead acid battery is dependent upon both the age and the temperature of the batteries. The use of ampere-hour meters cannot therefore provide a reliable indication of the energy remaining in the battery.